Mariana is the jilted ex-fiancé of Angelo (who dumped her after her dowry was lost when her brother's ship sank in the middle of the ocean). This, apparently, was pretty traumatic experience for Mariana because, when we're first introduced to her, she's isolated, living by herself in a farmhouse that's surrounded by a moat.
The most astonishing thing about Mariana is her willingness to participate in the bed trick that's devised by the Duke as a form of punishment for Angelo. On the one hand, we can see why Mariana might want to get back at Angelo. On the other hand, sleeping with Angelo means that Mariana will have to marry the guy who tossed her away because she was broke.
Brain Snack: In 1830, poet Alfred Tennyson (the guy who wrote "The Eagle" and "The Lady of Shalott") was so inspired by this character that he penned a poem called – you guessed it – "Mariana" (read it here).
A few years later, John Everett Millais (the artist responsible for Ophelia) produced a famous painting of Mariana in 1851 (check it out here). When the painting was first exhibited, it was accompanied by lines from Tennyson's poem:
She only said, "My life is dreary,
He cometh not," she said;
She said, "I am aweary, aweary,
I would that I were dead!"